Cell switching is a method of transmitting digital information wherein the information is broken into equal sized units called "cells." The cells of information are usually grouped into larger units called "frames."
The individual cells of information are transmitted from a source node to a destination node through a "connection." A connection consists of a pathway through a digital network. A digital network is constructed of digital switches coupled together by digital communication lines. At the destination node, the individual cells of information may be reconstructed into the frame.
Each cell originates at a source node and is transmitted across the communications lines. The communication lines carry the cells of information between the digital switches along the connection pathway. The digital switches route the cells from incoming communication lines to outgoing communication lines and finally to a destination node coupled to a digital switch.
Each digital switch in the network can be connected to several communication lines. Furthermore, each communication line can carry several different connections simultaneously. A digital switch may employ a set of queues to handle the connections and a queue server to serve the queues. For such digital switches, serving a connection with the queue server entails removing an incoming cell from an input queue associated with the connection and sending the cell out on a communication line or to a destination node coupled to the digital switch. A service algorithm is employed to select a connection for service. To fully utilize the bandwidth of a communication line, it must be possible to select a cell for service during each service time. The service time refers to the amount of time it takes to transmit a single cell onto the communication line. The objective of the service algorithm is to ensure that every connection receives the appropriate amount of service.
When a communication service provider agrees to provide service to a customer, the two parties enter into a communication traffic contract. This contract contains constraints that the customer agrees to adhere to. These constraints are referred to in this document as an arrival contract. For example, the arrival contract may limit the customer on how fast the customer may send cells to the network. Another part of the communication traffic contract may be used when a queue and queue server are used in the connection. For connections with a queue and queue server, the service provider may agree to provide the customer with a minimal level of service. This minimal level of service is referred to in this document as a service contract and may contain an initial burst and a minimum service rate.
To implement the arrival contract, the digital switch has a traffic policing circuit at the customer interface to the digital switch. The traffic policing circuit monitors the incoming cells sent from the customer to the service provider and determines when cells do not conform to the arrival contract. To implement a service contract, the digital switch has a service algorithm circuit that determines when the digital switch should service the queues for each connection that are handled by the digital switch. The service contract is not necessary when there is no queue associated with a connection.